The environmental impact of recreational diving is the effects of recreational scuba diving on the underwater environment, which is largely the effects of diving tourism on the marine environment. It is not uncommon for highly trafficked dive destinations to have more adverse effects with visible signs of diving's negative impacts due in large part to divers who have not been trained to sufficient competence in the skills required for the local environment, an inadequate pre-dive orientation, or lack of a basic understanding of biodiversity and the delicate balance of aquatic ecosystems. There may also be indirect positive effects as the environment is recognised by the local communities to be worth more in good condition than degraded by inappropriate use, and conservation efforts get support from dive communities who promote environmental awareness, and teach low impact diving and the importance of respecting marine life. There are also global coral reef monitoring networks in place which include local volunteer divers assisting in the collection of data for scientific monitoring of coral reef systems, which may eventually have a net positive impact on the environment.
During the 20th century recreational scuba diving was considered to have generally low environmental impact , and was consequently one of the activities permitted in most marine protected areas. Since the 1970s diving has changed from an elite activity to a more accessible recreation, marketed to a very wide demographic. To some extent better equipment has been substituted for more rigorous training, and the reduction in perceived risk has shortened minimum training requirements by several training agencies. Training has concentrated on an acceptable risk to the diver, and paid less attention to the environment. The increase in the popularity of diving and in tourist access to sensitive ecological systems has led to the recognition that the activity can have significant environmental consequences.
Scuba diving has grown in popularity during the 21st century, as is shown by the number of certifications issued worldwide, which has increased to about 23 million by 2016 at about one million per year. Scuba diving tourism is a growth industry, and it is necessary to consider environmental sustainability, as the expanding impact of divers can adversely affect the marine environment in several ways. The impact also depends on the specific environment; tropical coral reefs are more easily damaged by poor diving skills than some temperate reefs, where the environment is more robust and resilient due to rougher normal sea conditions and fewer fragile, slow-growing organisms. The same pleasant sea conditions that allow development of relatively delicate and highly diverse ecologies also attract the greatest number of tourists, including divers who dive infrequently, exclusively on vacation, and never fully develop the skills to dive in an environmentally friendly way. Various strategies for environmental management are being tested, in an attempt to achieve a sustainable balance between conservation and commercial exploitation.
Active avoidance of benthos contact requires appropriate motivation, and successful avoidance requires appropriate competence. Low impact diving training has been shown to be effective in reducing diver contact in suitably motivated divers. Experience appears to be the most important factor in explaining divers' underwater behaviour, followed by their attitude towards diving and the environment, and personality type.
All underwater environments frequented by recreational divers are potentially affected, but the impact is observed to be greater where there are large numbers of dives or the environment has fragile slow-growing organisms or delicate structures. The more obvious examples are tropical coral reefs and flooded caves with fragile speleothems. Relatively large numbers of studies have been done on tropical coral reefs as the pressure on them has been perceived to be highest. Coral reefs are to some extent self-repairing, and also sustain damage from non-anthropogenic causes, so an acceptable level of continuous diver induced degradation is theoretically possible, which would be matched by the natural recovery rate. Underwater speleothems, once broken, do not regenerate at all while the cave remains flooded, and remain broken forever, if human timespans are considered. Underwater cultural heritage in the form of historically important wrecks and archaeological sites are also susceptible to irreversible damage, but they are usually constantly deteriorating in any case. Diver impact mainly accelerates the inevitable.
Subtropical, temperate, and polar marine reef environments vary considerably in their ability to recover from damage, and have historically been considered less impacted by diving, consequently fewer studies are available on diver damage, reef recovery and sustainable carrying capacity.
Research on the effects of divers on tropical coral reefs has shown reduced coral cover on heavily dived sites and a change in coral structure, with more resilient corals becoming dominant and a loss of species diversity over time. These reefs may be less resilient to other stressors like disease outbreaks and severe weather damage.
There is persuasive evidence that reefs can be damaged and the amenity value of dive sites compromised by badly planned or over-intensive tourist use. Marine tourism affects reef communities directly through disturbance such as structural damage to corals, boats grounding on reefs and damage by anchors, and indirectly through alteration of water quality by nutrient enrichment and pollution by toxic substances, waste water and increased turbidity. The level of degradation depends on the intensity, frequency, time and type of use and the specific environment.
Diver impact damage to corals includes skeletal breakage of branching species, tissue abrasion, possibly leading to infection by coral diseases, and an overall reduction of hard coral coverage on reefs. Diving related activities may also reduce the reef's resilience to reef stressors like climate change and bleaching events.
In some frequently dived tropical coral reef sites recreational divers have caused negative ecological impacts by inadvertent impacts with live corals causing physical damage at a rate faster than compensated for by natural recovery. The long term result is reef degradation. One of the common challenges for local policy and management is maximising tourism benefits while also reducing environmental degradation to long-term sustainable levels.
In the soft sediment bottomed "muck diving" environment, it was observed that photography causes greater environmental disturbances than effects caused by diving experience, certification level, gender or age. Divers came into contact with the substrate more often on soft sediment than on coral reefs, but environmental damage was not greater. Divers tend to touch animals more frequently when observing or photographing cryptobenthic fauna, and spent up to five times longer in interactions when using dSLR-cameras. Long-term impacts of this behaviour on cryptobenthic fauna and soft sediment habitats are unknown.
The impacts of photographer behaviour and photographic flashes on a small sample of benthic fish species was investigated. The study showed negligible effects beyond those caused by human presence alone. Flash photography caused no discernible ocular changes in seahorses and feeding success was not affected. Physical handling of animals produced strong stress responses.
Diver impact on subtropical, and particularly temperate reefs, is less researched than tropical reefs. The perception is that these reefs are less vulnerable than tropical reefs and the sessile species are less exposed to diver impact. Research in the Mediterranean in Spain indicates that sessile organisms with fragile and brittle calcareous or corneous skeletons are not resilient to frequent disturbances by divers.
Diver contact with the bottom is also prevalent on temperate reefs, in freshwater environments, and in caves. One of the main forms mentioned is fin contact with the bottom sediment, raising particulate material to into the water column and degrading visibility, but disturbance of sediment and delicate benthic biofilms by fin wash without direct contact is also a concern. 
The impact of recreational scuba diving on recreational dive values and the cultural heritage of shipwrecks has been found to comprise four basic types:
Gender distribution of diver impact on reefs is inconclusive, different studies have produced contradictory findings.
Repetitive contact by divers and their equipment on the benthos is the general mechanism of reef degradation by recreational divers. Factors correlating with frequency of reef contact were found to be:
Observations that experience does not predict frequency of impacts is not necessarily incompatible with findings that practical training reduces frequency of impacts, as experience is not always an indicator of competence.
Fin impacts have been identified as contributing the most to damage to reef biota; erect and branching hard corals are the most sensitive taxon to contact damage on tropical reefs, and the severity of damage is influenced by habitat complexity. This indicates that better diver trim, buoyancy and finning techniques, situational awareness of position relative to the reef, and awareness of the damage done by contact with corals in habitats where close proximity of fins to sensitive organisms is likely, are priorities for reducing damage. Several studies have found that damage to coral reefs by divers can be minimized by modifying the behavior of those divers. Training in low impact diving skills appears to significantly reduce contact with the benthos in divers of all certification and experience levels. This result can be extrapolated to other diving environments as a method to protect the environment and help to make recreational scuba diving more ecologically sustainable, and may enhance the diving experience.
There appears to be little correlation between site topography and coral damage, but damage is related to coral morphology and structural strength. Most damage is to branching species which are inherently weaker against bending loads.
Several studies have found the main reason for contact by inexperienced divers to be poor buoyancy control. Studies on recreational divers on tropical coral reefs have shown that the rate of contact between diver and environment varies significantly between divers who are able to maintain neutral buoyancy and those who are deficient in the skill, with divers who do not maintain neutral buoyancy contacting the reef more often. Briefing divers on the effects of contact with the reef reduced contact in divers with good buoyancy skills, but not in divers who lacked those skills. The problem appears to be one of competence. Without the necessary competence, divers are unable to modify their behaviour appropriately, and cannot produce the skills merely by being made aware of their necessity. The solution to reducing reef contact is in requiring the diver to have the skill before allowing them to dive in the environment where it is needed.
There is evidence that the ability of dive guides to positively influence diver behaviour relating to reef contact is less for larger groups of divers, but the implementation of programmes which focus on dive industry operations can contribute to the reduction of anthropogenic reef damage.
Some MPAs in the Mediterranean have prohibited scuba diving completely, or have restricted it to reefs near the boundaries of the MPA. Others have established diving trails which keep divers away from vulnerable areas. Another conservation strategy identified potentially vulnerable species and based the determination of sustainable numbers of visitors on this.
A 2012 study by Ong and Musa on recreational divers in Malaysia showed that experience was the most important factor in explaining divers’ underwater behaviour, followed by the attitude towards diving and personality type, and that attitude towards the environment partially mediates the influence of experience on underwater behaviour, but Scott-Ireton (2008) found that an awareness of how and why the environment should be protected has more effect than skill competence for motivating divers to conserve their surroundings. Active avoidance of reef contact requires appropriate motivation, and successful avoidance requires appropriate competence.
In 1989, Buoyancy Training Systems International, a company based in Seattle, Washington, became the first organization in the world to create an internationally uniform, training and objective underwater test of skill specifically designed to reduce diver impact upon the marine environment. The curriculum and mobile practice venue, now known as the Diamond Reef System, uses standardised portable reef simulation structures called ‘Diamond Reef Hover Stations' to raise the proficiency and awareness of divers at all stages of diver training including tropical resort acclimation dives. This program remains in use by dive operators world-wide and the U.S. Environmental Protection Agency. 
The specific PADI Low Impact Diver training program takes 2 days and appears to be effective for a large range of pre-existing skill and certification levels. Similar training from other providers should have similar results. Many of the skills are included in technical diver training, particularly cave and wreck diving, where they are also important for safety.
While competence, which is directly associated with training and experience, is fundamentally necessary for low impact diving, attitude towards the environment and personality type appear to influence whether and how the diver applies their competence to moderating ecological impact.
Several researchers have found evidence to indicate that much of the damage to the underwater environment could be avoided by modifying the behaviour of divers by a combination of an education session followed by in-water demonstration, but short pre-dive briefings alone have little effect on contact rates.
Camp and Fraser, 2012, found that divers who had taken part in environmental conservation courses contacted the reef as often as those who did not, but conservation education provided in a dive briefing reduced contacts with the reef, and the depth of conservation education is relevant to its effectiveness. Diver familiarity with the local environment was also mentioned as having an effect on reducing impact frequency.
There are a variety of scuba diving programs that provide enriching instruction into not only proper techniques and equipment but also a focus on environmental conservation, specifically preservation of coral reefs. For instance, in the Florida Keys, companies such as Quiescence and Sea Dwellers have several educational opportunities for advanced open-water divers to learn more about the impact of the recreational sport on ocean wildlife, as well as have affiliations with organizations such as the Blue Star Dive Center and the Coral Restoration Foundation. These programs emphasize the importance of divers maintaining relative distance from themselves and sea creatures to minimize contamination and preserve the health and safety of the underwater world. In particular, the Coral Restoration Foundation provides a thorough illustration of the essential role that coral reefs play in the ocean environment, as a source of nutrients and sustainability for about a quarter of all aquatic life. Although people tend not to think of coral reefs as animals, each coral species relies on safe interaction with other organisms, the energy they receive from the algae zooxanthellae, and the ability to thrive in an ecosystem free from harmful contaminants. Unfortunately, over recent years as escalating carbon dioxide emission and the resulting climate change crisis have posed an increasing threat, many coral reefs around the world have suffered from the effects of rising temperatures, which have destroyed the growth of zooxanthellae that coral polyps regularly depend on to survive.
In addition to these smaller Florida organizations addressing the environmental problems associated with unregulated scuba diving practices, other international groups have expressed similar concerns that this sport can potentially harm marine life. For instance, Kish Island along the Persian Gulf has experienced a wave of water sport activity, particularly scuba diving and snorkeling, and while the coral reefs present around the land mass are often safely viewed by divers, certain sensitive species can be negatively impacted by this constant human interaction. According to the Regional Studies in Marine Science journal, divers and snorkelers can cause damage to intertidal and shallow parts of reefs, posing a detriment to the survivability of both the coral and the organisms that depends on its resources and nutrients. In order to ensure that the industry engaged with diving tourism does not directly or indirectly threaten the existence of this essential aquatic life, it is extremely important that diving sites uphold proper carrying capacities to reduce overpopulated diving expeditions and also that researchers investigate more deeply into studying the exact effects of recreational diving at Kish Island and other places containing rich oceanic wildlife.
Several methodologies have been developed with the intention of minimising the environmental impact of divers on coral reefs
The number of dives over a time interval that results in an acceptable and stable level of degradation depends on a combination of factors that vary between sites. These include the presence of vulnerable organisms, and the stresses to which they are specifically vulnerable, the level of environmental awareness and responsibility of the divers, the diving skills and situational awareness of the divers, prevalent water conditions during dives, including currents and surge which increase difficulty of positional control, other anthropogenic stressors that may be present, and may combine their effects with diver impact, topographical details which affect diver maneuvering and risk of impact, and the size of the site. Continued periodical re-assessment will generally be necessary, even when there are no obvious large changes to external circumstances. These factors may vary considerably even within a specific ecological zone, and may be expected to be very different between for example, a tropical coral reef dominated by branching stony corals on a gradually sloping bottom with persistent moderate wave action and continuous current, in an area far from major industry, and a high profile granite corestone temperate reef with deep gullies and steep ridges, dominated by kelp, ascidians and echinoderms, and with seasonal variations in prevailing wave direction near a large industrial city. The appropriate equipment and relevant skills will also vary, and unfamiliar rental equipment can require some practice before the diver can use it well, particularly for optimising weighting and trim. Divers in training can not be expected to perform at their best while distracted by learning new skills, and this is also a factor to be considered as a potential stressor if there is no separation between training sites and sites for qualified divers.
Recreational diver activities with an overall positive impact on the environment may include citizen science reef monitoring by volunteer recreational divers and species observation reports by underwater photographers, economic pressure by diving tourists to conserve desirable diving conditions, and cleanup of plastic and other undesirable debris by divers. Most of the citizen science projects require some level of long term commitment, including training, and reasonably frequent activity to maintain skills, and are therefore more suitable for local residents than for tourists.
Marine conservation projects using volunteer recreational divers:
Observational record databases for science and conservation: